In ultrasonic testing of pipe or tube (Hereinafter, “pipe or tube” is referred to as a pipe when deemed appropriate.), it is necessary to detect not only an axial flaw (a flaw extending in the axial direction of pipe) but also a circumferential flaw (a flaw extending in the circumferential direction of pipe) and a planar flaw parallel with the inner and outer surfaces of pipe called a lamination.
The axial flaw is detected by transmitting and receiving ultrasonic waves by using an ultrasonic probe 1′ tilted in the circumferential direction of a pipe P and by propagating ultrasonic shear waves in the circumferential direction of the pipe P as shown in FIG. 1A.
Also, the circumferential flaw is detected by transmitting and receiving ultrasonic waves by using an ultrasonic probe 1′ tilted in the axial direction of the pipe P and by propagating ultrasonic shear waves in the axial direction of the pipe P as shown in FIG. 1B.
Further, the lamination is detected by propagating ultrasonic longitudinal waves in the wall thickness direction of pipe as shown in FIG. 1C.
Thus, to detect various flaws that extend in different directions or have different shapes, a plurality of ultrasonic probes are generally used according to the type of flaw.
Conventionally, for a flaw of any of the types mentioned above, as shown in FIG. 2, ultrasonic testing has been performed by using a plurality of ultrasonic probes 1′ (in the example shown in FIG. 2, four ultrasonic probes denoted by symbols A1 to A4) arranged along the axial direction of pipe to enhance the testing efficiency and by transmitting and receiving ultrasonic waves from/by the ultrasonic probes 1′ and conveying the pipe in the axial direction while the pipe is rotated in the circumferential direction (spiral conveyance). For example, the ultrasonic probe A1 tests the pipe in a spiral region hatched in FIG. 2, and the ultrasonic probe A2 tests the pipe in a spiral region shaded in FIG. 2. Thus, by arranging the plurality of ultrasonic probes 1′ along the axial direction of pipe, whole surface testing of pipe has been realized.
However, the conventional ultrasonic testing method shown in FIG. 2 has a problem that the flaw detectability is deteriorated because of the decrease in sound field intensity at the boundary of sound fields formed by the adjacent ultrasonic probes 1′. FIG. 3 shows a profile example of echo intensity from a circumferential flaw formed on a pipe by machining. Specifically, FIG. 3 is a graph in which the flaw echo intensities obtained with the ultrasonic probes A1 to A4 by moving the pipe, on which a circumferential flaw has been formed, in the axial direction correspond to the axial location of the circumferential flaw. FIG. 3 reveals that in the boundary portion of the adjacent ultrasonic probes 1′ (the portion encircled by a broken line in FIG. 3), the flaw echo intensity decreases markedly. Therefore, it is apparent that the flaw detectability is also markedly deteriorated in this boundary portion.
To solve the above problem, for example, a technique described in JP3674131B (Patent Document 1) has been proposed. In the technique described in Patent Document 1, a plurality of ultrasonic probes are arranged in a straight line, and from among the plurality of ultrasonic probes, a probe group consisting of a fixed number of continuous ultrasonic probes is selected to transmit and receive ultrasonic waves, the probe group to be selected being switched successively. This technique is characterized in that the switching pitch is matched to the total width of a plurality of ultrasonic probes, and is set so as to be equal to or smaller than the practical effective beam width of the ultrasonic wave emitted from one selected probe group.
The “practical effective beam width” is defined as a width until the level is decreased by 3 dB with respect to the peak value of sound field intensity obtained in the central portion of ultrasonic probe (paragraph 0005 of Patent Document 1). That is to say, the “practical effective beam width” is a value determined from the profile of sound field intensity of the ultrasonic beam transmitted from the ultrasonic probe.
However, when the technique described in Patent Document 1 is applied to testing of a pipe whose various flaws extend in different directions or have different shapes must be detected, a problem described below arises.
The profile of flaw echo intensity obtained when the ultrasonic probes are scanned in the arrangement direction thereof (in the example shown in FIG. 2, the axial direction of pipe) cannot be determined uniquely by only the profile of sound field intensity, and the flaw shape in the scanning direction of the ultrasonic probe exerts a great influence.
FIG. 4 shows a profile example of flaw echo intensity obtained when the identical ultrasonic probe is scanned in the axial direction of pipe with respect to an axial flaw and a circumferential flaw produced on the pipe.
In the above-described example, since the identical ultrasonic probe is used, the profile of sound field intensity is the same. However, as shown in FIG. 4, if the flaw is different, the profile of flaw echo intensity will be different. Therefore, with the practical effective beam width derived from the profile of sound field intensity, the switching pitch cannot be determined properly, and the flaw may be overlooked. Consequently, there arises a problem that the switching pitch must be determined inevitably by repeating trial and error for each type of flaw.